Method for removing metal contained in solution using surfactant having chelating ability

ABSTRACT

Disclosed herein is a technique of removing metal contained in a solution through chelation, which makes it possible to preclude undesired aggregation and permit effective aggregation through an aggregating process. A surface active agent containing a hydrophobic group, a hydrophilic group and a chelating group is added to the solution to be processed, and subsequently an aggregating process is carried out. The chelating group chelates metal, the hydrophilic group prevents undesired aggregation, and the hydrophobic group is effectively aggregated in the aggregating process. Various uses are possible by selecting the charge of ligand.

This is a division of application Ser. No. 08/527,206 filed on Sep. 12,1995, now U.S. Pat. No. 5,587,060.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a technique of removing metal contained in asolution and, more specifically, it provides a method of and anapparatus for removing metal as well as a surface active agent(surfactant) having chelating ability and used suitably for such methodand apparatus. In addition, this invention relates to a technique ofseparating the removed metal from the solution. Here, the term "removalof metal from a solution" refers to "chelation of metal" and does notnecessarily refer to "separation of metal from a solution".

2. Description of the Prior Art

Japanese Laid-Open Patent Publication No. 2-274900 discloses a processof separating metal ions from a solution. This technique employs achelating agent which chelates metal ions to generate insoluble matterin the solution. More specifically, in this technique, the chelatingagent noted above is added to the solution containing metal ions,followed by filtering out the insoluble matter to separate metal ionsfrom the solution. The chelating agent used here does not contain anyhydrophilic group because it is necessary to generate insoluble matterafter chelating metal ions.

The technique in which the chelating agent free from hydrophilic groupis used to chelate metal ions and generate insoluble matter, therebyseparating metal ions from the solution, has the following problems. Oneof the problems is that the insoluble matter that is generated by thechelating agent is usually very fine in grain size and is difficult tobe filtered out. Another problem is posed when re-using the solution.That is, during re-use of the solution which contains chelating agentadded excessively for metal removal, insoluble matter is generated. Forexample, during washing of a plated product, metal ions are dissolved inthe washing solution. If this washing solution is re-used without anyprocessing, the metal ions therein are attached to a plated product. Forthis reason, it is necessary to recover the washing solution and removemetal ions therein. At this time, it is difficult to add an amount ofchelating agent which is neither excessive nor insufficient with respectto the amount of metal ions in the solution. In the case of the JapaneseLaid-Open Patent Publication No. 2-274900 noted above, it is recommendedto add double the theoretically necessary amount of the chelating agentto increase the metal ion capturing factor. In such case, it isinevitable that ligands which do not chelate metal ions remain in thewashing solution supplied for re-use. Consequently, metal ions dissolvedin the washing solution are progressively chelated by ligands which havenot chelated metal ions, thus resulting in the generation of insolublematter during the washing.

SUMMARY OF THE INVENTION

The present invention seeks to create a technique which does not lead toaggregation through mere chelate removal of metal but leads to effectiveaggregation by carrying out a separate processing for aggregation, thussolving the problems inherent in the prior art.

According to the invention, a chelating agent is used. The chelatingagent, however, contains a hydrophilic group so that it is notaggregated through mere chelation of metal. In addition, it contains ahydrophobic group so that it can be effectively aggregated through anaggregating process. Some surface active agents (surfactants) whichcontain a hydrophilic group, a hydrophobic group and a chelating group,are known in the art. However, the known surface active agents aremostly used as dispersing agent but are not used for removing metalcontained in solution.

According to the invention, a surface active agent (surfactant)containing a hydrophilic group, a hydrophobic group and a chelatinggroup, is added to a metal-containing solution to be processed. As aresult, metal contained in the solution is chelated or removed by thechelating group. However, since the surface active agent contains thehydrophilic group, no precipitate is generated through mere chelation ofmetal. If the amount of surface active agent used is exceeding themicelle limit, micelles are formed in the solution. However, suchmicelles are not precipitated. Thus, mere dissolution of metal in thesolution excessively containing surface active agent does not lead toprecipitate generation.

The surface active agent which contains the hydrophilic group, thehydrophobic group and the chelating group having chelated metal, formsmicelles with the hydrophobic group inside, but no precipitate isgenerated. However, when an electric field is applied to this solution,the micelles therein are aggregated to grow into a large mass. Otherprocesses for aggregation may be adopted.

According to a method of the invention, in a first step, a surfaceactive agent (surfactant) containing a hydrophilic group, a hydrophobicgroup and a chelating group, is added to a metal-containing solution, sothat the surface active agent forms micelles with the hydrophobic groupinside and at the same time captures metal with the chelating group onthe micelle particle surface. As this occurs, the metal is removedsubstantially from the solution. In a second step, an electric field isapplied to the solution, so that micelles are aggregated into a largemass. The large mass of micelles thus formed is separated, so that themetal can be separated effectively from the solution.

This technique is applicable as well to the removal of metal ions froman aqueous solution in which oil is emulsified, for instance, an aqueouscutting oil in which mineral oil is emulsified. Unless the metal ionsare removed, the surface active agent effective to emulsify the oiltends to be deteriorated and eventually separated into water and oil. Inthis case, a surface active agent containing a hydrophilic group, ahydrophobic group and a chelating group is selected for emulsifying oil,the chelating group of which has a ligand with a negative electriccharge not in just canceling relation to the positive electric charge ofmetal ions to be chelated. When oil has been emulsified by the surfaceactive agent containing the chelating group having the electric propertyas noted above, metal ions dissolved in the aqueous solution can beeffectively captured by the chelating group and removed from the aqueoussolution. Since at this time, the chelating group which is dissociatedin the solution has a negative electric charge not in just cancelingrelation to the positive electric charge of metal ions, electric chargeremains in the chelating group after capturing the metal. The emulsionparticles which are formed by the surface active agent dispersing oiland oil particles are thus charged and electrically repel one anotherwithout being aggregated, so that they exist in a stable state in thesolution. Thus, according to the invention, it is possible to removemetal ions from the aqueous solution without spoiling the emulsifiedstate of oil. It will be noted that the solution of which the metal isremoved and the removed metal can be separated by a separate process.

To effectively separate metal from a solution without aggregationthrough mere removing of metal while permitting effective aggregationthrough an aggregating process, a surface active agent is suitably used,which contains a hydrophilic group, a hydrophobic group and a chelatinggroup, and in which the chelating group contains a ligand with anegative electric charge in just canceling relation to the positiveelectric charge of metal ions to be chelated. With this surface activeagent, the chelating group becomes electrically neutral when metal ionsare captured, thus permitting very efficient aggregation through asubsequent aggregating process. Even with the chelating group becomingelectrically neutral as a result of metal ion capturing, aggregation isnot readily caused by the hydrophilic group, that is, undesiredaggregation does not occur during use of the solution. As describedbefore, there are some known surface active agents which contain thehydrophilic group, the hydrophobic group and the chelating group (theirpurpose being not for metal removal). However, the surface active agentcontaining a ligand with a negative electric charge in just cancelingrelation to the positive electric charge of metal ions to be chelated isprovided as creation according to the invention. The electric charge ofmany kinds of metal ions are two-valent or above, while the electriccharge of the prior art surface active agents have been limited to beuni-valent. The surface active agent according to the invention featuresthat the electric charge is two-valent or above.

When a solution contains metal such as Cu, Pb, Ag and Hg, in order toremove such metal from the solution, it is preferable to use a surfaceactive agent (surfactant) which comprises a hydrophilic group, ahydrophobic group and a chelating group, the chelating group containinga ligand in which sulfur atom is a coordinate atom. The surface activeagent can also be used for emulsifying oil in an aqueous solution.

When the coordinate atom is sulfur atom, Cu, Pb, Ag or Hg is effectivelychelated and removed from the solution, while Fe or Al is hardlychelated. For this reason, of metals, Cu, Pb, Ag and Hg can selectivelybe chelated. Thus, this technique is effective when there is less needto remove Fe or Al and substantial need to remove Cu, Pb, Ag or Hg.

Further, the invention provides a surface active agent which contains ahydrophobic group, an ethylene oxide group bonded to the end of thehydrophobic group, and a chelating group containing a multiple seatligand bonded to the ethylene oxide group. This surface active agent hasa hydrophilic group which is strongly hydrophilic owing to the ethyleneoxide group, and aggregation does not readily result from mere chelationof metal. Thus, when metal ions are dissolved in re-used solution withthe surface active agent excessively added thereto, it is possible topermit capturing of the metal without any aggregation. Thus, theintrinsic property of the solution is not spoiled.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects, features and advantages of the invention will becomemore apparent from the following detailed description of the preferredembodiment when the same is read with reference to the accompanyingdrawings, in which:

FIG. 1 is a view showing an embodiment of the invention applied to aliquid regenerating system;

FIG. 2 is a schematic view showing a micelle;

FIG. 3 is a view showing micelles before aggregation;

FIG. 4 is a view showing micelles after aggregation;

FIG. 5 is a view showing a different embodiment of the invention appliedto a liquid regenerating system;

FIG. 6 is a view showing an example of chemical structure of a surfaceactive agent containing a hydrophilic group, a hydrophobic group and achelating group;

FIG. 7 is a view showing a different example of chemical structure of asurface active agent containing a hydrophilic group, a hydrophobic groupand a chelating group;

FIG. 8 is a view showing a further example of chemical structure of asurface active agent containing a hydrophilic group, a hydrophobic groupand a chelating group;

FIG. 9 is a view showing a still further example of chemical structureof a surface active agent containing a hydrophilic group, a hydrophobicgroup and a chelating group;

FIG. 10 is a view showing a still further example of chemical structureof a surface active agent with improved hydrophilic power; and

FIG. 11 is a view showing an example of a surface active agent in whichsulfur atom is a coordinate atom.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Examples of SuitableSurface Active Agent

Type 1: Charge Neutralizing Type

This type of surface active agent is used suitably when importance isattached to effective aggregation in an aggregating process. The surfaceactive agent has a ligand with a negative electric charge in justcanceling relation to the positive electric charge of metal ions to bechelated. The surface active agent is obtained by substituting a memberof the group consisting of hydroxyethyl (--CH₂ --CH₂ --OH),carbamylethyl (--CH--CH₂ --CO--NH₂), and carboxymethyl (--CH₂ --COOH)with or without a bonded alkylamino group, the substituent correspondingin number to the valence of electric charge of metal ions to be chelatedfor one amino nitrogen atom at the end of a long chain alkyl amino groupR with a carbon number of 8 to 30 or two amino nitrogen atoms spacedapart by ethane or propane. Examples of this type of surface activeagent are shown in FIGS. 6 and 7. Examples of a similar surface activeagent are shown in FIG. 9.

For example, when all the metal ions contained in a solution aretwo-valent (such as Cu⁺⁺, Ni⁺⁺ and Zn⁺⁺), a surface active agent isused, which is obtained by substituting two chelating groups noted abovefor one or two amino nitrogen atoms at the end of the long chain alkylgroup R. When all the metal ions contained in a solution arethree-valent (such as Al⁺⁺⁺, Cr⁺⁺⁺ and Fe⁺⁺⁺), a surface active agent isused, which is obtained by substituting three chelating groups for twoamino nitrogen atoms. In this case, the chelating groups to besubstituted for amino nitrogen may be of the same kind or of differentkinds and are to be selected in dependence of the character of thesolution to be processed. It is further possible to substitute fourchelating groups. Such an example is shown in FIG. 8.

The prior art surface active agents containing the chelating group havea ligand with negative electric charge with single valence. Therefore,they are charged positively when they chelate two- or three-valent metalions, that is, neutralization of charge is not obtained.

The surface active agent of the type of this example is designed toavoid mutual electric repulsion of emulsion particles even with chelateformation and at the same time, to permit effective aggregation onlythrough an aggregating process.

This type of surface active agent tends to be readily aggregated andseparated after metal is removed from the solution. Thus, the surfaceactive agent, when added to an oil removal bath of oil separation type,forms stable chelate with metal in the bath to be shifted to the floatedoil layer. Metal thus can be separated with mechanical separation of theoil layer.

Type 2: Strongly Hydrophilic Type

This type of surface active agent comprises an hydrophobic group such ashigher alcohol and long chain alkylphenol, an ethylene oxide groupbonded to the hydroxyl group (the end of the hydrophobic group) and achelating group having a multiple seat ligand coupled to the end of theethylene oxide group. The chelating group is suitably a member of thegroup consisting of hydroxyethyl, carbamylethyl and carboxymethyl asnoted above with or without bonded alkylamino nitrogen. An example ofthis type of surface active agent is shown in FIG. 10.

This type of surface active agent is strongly hydrophilic owing to theethylene oxide group, and no aggregation is brought about as a result ofmere capturing of metal by its chelating group. However, it is possibleto attain the aim of aggregation through an aggregating process in whichan electric field is applied.

This type of surface active agent is effective as softening agent(sealing agent such as Ca⁺⁺, Mg⁺⁺, Fe⁺⁺, Fe⁺⁺⁺ and Al⁺⁺⁺) for generalfat removal bath. Almost all fat removal baths are alkaline baths inwhich introduced metal ions are present as oxides or hydroxides and maybe re-attached to the surface of the object being washed. At present, toprevent this, the introduced metal is made soluble using EDTA or likeagents. This, however, poses a problem of non-precipitation of metalwhen the solution is processed for drainage from the bath. The surfaceactive agent of the type in this case seals the harmful metal withstrong chelating force. The removed or chelated metal may be separatedby a charging process.

Further, the instant surface active agent is suited for removing metals(such as ions, oxides and hydroxides) in water washing baths of platingor coating solutions. In the existing precipitating process using achelating agent (shown in Japanese Laid-Open Patent Publication No.2-274900), there are harms of chelating agent present in the solution.This problem can be solved by using the instant surface active agent.That is, not only the surface active agent but also the generatedchelate of metal are present as emulsion (i.e., hydrophilic colloid).Thus, re-attachment of the precipitate to the surface of the objectbeing washed does not take place, and the metal-containing emulsion canbe separated by the charging process. It is thus possible to realizecontinuous use of the water washing bath.

Type 3: Emulsion Stabilizing Type

This type of surface active agent is used effectively for removingmetals (such as ions, hydroxides and oxides) present in aqueoussolutions in which oil is emulsified without spoiling the emulsifiedstate of the oil. As an example, when an aqueous cutting agent is usedto cut metal, an oxide, a hydroxide or ions of the metal is increasedconsiderably in the solution. In this case, an anionic surface activeagent which is supporting the oil emulsification forms an insoluble saltwith the compound of the metal and is eventually deteriorated. By usingthe instant type of surface active agent, chelation and emulsificationare brought about prior to the formation of an insoluble salt by theharmful metal compound with the emulsion-supporting surface activeagent, thereby maintaining the emulsified state of the oil. Themetal-containing emulsion can be separated from the aqueous solutionthrough a separate charging process of the aqueous solution. In thiscase, as the chelating group of the surface active agent used isselected those which contain a ligand with an electric charge not injust canceling relation to the positive electric charge of the metalions.

The chelating group introduced into the surface active agent molecule,such as --CH₂ --COOH, --CH₂ --CH₂ --CO--NH₂ and --CH₂ --CH₂ --OH hasvariable chelate formation function in dependence on the character(acid, neutral, or alkaline) of the solution processed. Thus, this grouphas to be selected in dependence on use.

Type 4: Cu or Pb Containing Solution Processing Type

This type of surface active agent comprises a hydrophilic group, ahydrophobic group and a chelating group, the chelating group containinga ligand in which sulfur atom is a coordinate atom. Examples of thistype of surface active agent are shown in FIG. 11. In the drawings, Rdesignates an alkaline group having 7 or more carbons. n is an integralnumber not less than 1. The ligand containing sulfur atom effectivelychelate Cu, Pb, Ag or Hg. For this reason, the surface active agent issuitably used for processing washing solution of electrodepositioncoated products containing much Cu or Pb. In a solution containing Fe,Al, Pb, Cu, Zn and other metals, there may exist less need to remove Feand Al and substantial need to remove Pb, Cu, Zn and other metals. Itwill be noted that the chelating group of the surface active agent isless effective to capture Fe and Al. For this reason, when this type ofsurface active agent is used in the case noted above, the chelatingagent will not wastefully be consumed for Fe and Al of which need forremoval is less, thus making it possible to use minimum chelating agentas required.

Embodiment Concerning Water Generation System

An embodiment of the invention applied to a system for washing a platedproduct 3 is shown in FIG. 1. The plated product 3 requires washing, andis washed in a washing tank 4. Washing solution overflowing from thewashing tank 4 is recovered in an overflow tank 5. Surface active agenthaving the chelating function as described above is added to the washingsolution in the overflow tank 5 from surface active agent adding means2.

The solution recovered in the overflow tank 5 is transferred by a pump 6to a high frequency electric field aggregator/separator 1. In the highfrequency electric field aggregator/separator 1, a high frequencyelectric field at 1 kHz or above is applied to the solution.

While the plated product 3 is washed in the washing tank 4, metal ionsare dissolved in the washing solution from the plated product 3. Thedissolved metal ions are chelated by the added surface active agenthaving the chelating function. At this time, if the concentration of themetal ions is high, the hydrophobic groups of the surface active agenthaving chelated metal ions are bonded together to form micelles 20 asschematically shown in FIG. 2. Designated at 22 in FIG. 2 arehydrophobic groups, at 24 hydrophilic groups, and at 26 chelating groupsafter chelation of metal. The micelles 20 are very fine. Nevertheless,they are not precipitated in the overflow tank 5 because of their highhydrophilicity owing to the hydrophilic groups 24. Further, they are notprecipitated in the washing tank 4 even when the plated product 3 iswashed with excessive chelating agent added to the washing solution.

The washing solution and the surface active agent are agitated by thepump 6 for efficiently chelating metal ions, and the resultant mixtureis supplied to the aggregator/separator 1 in the state that micelleshave been formed by the bonding of hydrophobic groups. The result ofmicroscopic observation of this solution in this state is shown in FIG.3. Micelles 20 in the solution are vibrated finely in the high frequencyelectric field set up in the aggregator/separator 1. As a result of thevibrations, the micelles 20 are aggregated. The result of microscopicobservation after the aggregation is shown in FIG. 4. The aggregateresulting from the aggregation of micelles is floated to the surface ofthe solution to be recovered in a recovery tank. After the recovery ofthe aggregate, the washing solution from which metal ions have beenseparated is returned to the washing tank 4.

The washing solution regenerating process may be carried out as batchprocess as shown in FIG. 5. In this case, when the metal ionconcentration in the washing solution has been increased, the solutionis transferred by a pump 6a to an intermediate set tank 8. Then, theprocessed washing solution is supplied by a pump 6c from a surfaceactive agent adding tank 9 to a washing tank 4 to continue the washingprocess. When the processed washing solution is transferred from thetank 9, washing solution having high metal ion concentration istransferred by a pump 6b to the tank 9 to be processed in the tank 9.

In the process, first the surface active agent having the chelatingfunction is added from the surface active agent adding means 2 to thesolution. The resultant mixture is then supplied by a pump 6d to theaggregator/separator 1, and aggregated surface active agent havingchelated metal ions is recovered in a recovery tank 7. From thissolution, metal ions are separated. After the removal of metal ions, thesolution is returned to the tank 9. In this way, with increase of themetal ion concentration of the washing solution in the washing tank 4,the solution is renewed, and this operation is repeated.

The high frequency electric field aggregator/separator 1 includes a tankand at least one pair of electrode plates secured to the inner walls ofthe tank, a high frequency voltage being applied between the electrodeplates. While this embodiment is applied to the processing of washingsolution, the invention is also applicable to the processing of platingsolution, water-soluble cutting oil, etc.

As has been described in the foregoing, with the metal removal techniqueaccording to the invention, metal is removed from a solution by using asurface active agent which has a chelating group for capturing metal, ahydrophilic group serving to prevent precipitation as a result of merecapturing of metal, and a hydrophobic group for providing for effectiveaggregation in an aggregating process. It is thus possible to obtaineffective metal removal while maintaining the intrinsic property of thesolution without being spoiled. Besides, when the solution is putthrough a special separating process, the solution of which metal isremoved and the removed metal can be readily separated.

In addition, when the surface active agent with chelating function usedaccording to the invention has a negative electric charge not in justcanceling relation to the positive electric charge of metal ions to bechelated, it is possible to obtain effective metal removal withoutspoiling emulsion property of oil drops.

Further, a surface active agent which has a negative electric charge injust canceling relation to the positive electric charge of metal ions tobe chelated, can be effectively aggregated after chelating metal, thusobtaining improved aggregating and separating functions.

Furthermore, the use of the surface active agent containing ion atom asa ligand makes it possible to effectively remove metal such as Cu, Pb,Ag and Hg.

Moreover, a surface active agent making use of the ethylene oxide grouphas a strongly hydrophilic group and is not aggregated unless anaggregating process is carried out. It is thus possible to preventundesired aggregation.

What is claimed is:
 1. A method of removing metal from an oil emulsifiedaqueous solution, comprisingemulsifying an aqueous solution, with asurface active agent containing a hydrophilic group, a hydrophobic groupand a chelating group, the chelating group containing a ligand with anegative electric charge not equal to the positive electric charge ofmetal ion to be chelated by the chelating group, wherein the electriccharge is not neutralized even after the metal chelation.
 2. The methodof claim 1, wherein said metal ion is selected from the group consistingof Cu, Pb, Ag, and Hg.
 3. The method of claim 1, wherein the surfaceactive agent contains a hydrophobic group, an ethylene oxide groupbonded to the end of the hydrophobic group and a chelating group havinga multiple seat ligand bonded to the end of the ethylene oxide group. 4.The method of claim 1, wherein the surface active agent contains aligand in which a sulfur atom is a coordinate atom.